The applicability of leak-before-break on flooded member detection for offshore structures under fatigue

The full development of growth of a circumferential crack in a tubular member from a long deep surface crack to final failure has been investigated by carrying out fracture mechanics based fatigue analyses, including the stage of a partial through-wall crack. The influences of member size, crack configuration material properties and loading conditions on the fatigue life have also been explored. The time available to detect the presence of the crack once it has penetrated the wall thickness (the flooded life) depends particularly on the rate of accumulation of fatigue damage (S3N/year) and on the initial length of surface crack present. The analysis results have been applied to a case study of a particular offshore structure and loading situation and found to support inspection intervals of three years for flooded member detection for this case. The results of this fatigue analyses, have been compared to a conventional leak-before-break (LBB) procedure in flooded member detection. It was found that, for long deep cracks, the standard LBB procedure may underestimate the flooded life available for inspection, but for short cracks a standard LBB approach using extrapolations of standard stress intensity factor solutions would be non conservative.